Q39.Q40.Q41.Q42.Q43.Q44.Q45.What is the most important thing you shouldknow before you perform a diagnosis on a systemproblem?A refrigerant leak appears in what way at thepoint of the leak?What is the most common cause of leaks onautomotive air-conditioning systems?For a refrigerant to be a suitable flushing agent,it must remain in what state during flushingoperations?Which part of an automotive air-conditioningsystem is replaced because it is impossible toclean?A type IV certification is also known as what typeof certification?Who approves organizations to certifytechnicians?DUCTWORKLearning Objective: Understand the basic typesof ductwork systems and the components of thosesystems for distribution of conditioned air.Distributed air must be clean, provide the properamount of ventilation, and absorb enough heat to coolthe conditioned spaces. To deliver air to theconditioned space, air carriers are required, which arecalled ducts. Ducts work on the principle of airpressure difference. If a pressure difference exists, airwill flow from an area of high pressure to an area of lowpressure. The larger this difference, the faster the airwill flow to the low-pressure area.CLASSIFICATION OF DUCTSThere are three common classifications ofductsconditioned air ducts, recirculating-air ducts,and fresh-air ducts. Conditioned air ducts carryconditioned air from the air conditioner and distributeit to the conditioned area. Recirculating air ducts takeair from the conditioned space and distribute it backinto the air conditioner system. Fresh air ducts bringfresh air into the air-conditioning system from outsidethe conditioned space.Ducts commonly used for carrying air are of around, square, or rectangular shape. The most efficientduct is a round duct, based on the volume of air handledper perimeter distance. In other words, less material isneeded for the same capacity as a square or rectangularduct.Square or rectangular duct fits better to buildingconstruction. It fits above ceilings and into walls and ismuch easier to install between joists and studs.TYPES OF DUCT SYSTEMSThere are several types of supply duct systems (fig.7-42) that deliver air to room(s) and then return the airfrom the room(s) to the cooling (evaporator) system.These supply systems can be grouped into four types:1. Individual round pipe system2. Extended plenum system3. Reducing trunk system4. Combination (of two or more systems)Return air systems are normally of threetypessingle return, multiple return (fig. 7-42), orcombination of the two systems.CONSTRUCTIONDucts may be made of metal, wood, ceramic, andplastic. Most commonly used is sheet steel coated withzinc (galvanized steel). Sheet metal brakes andforming machines are used in fabricating ducts.Elbows and other connections, such as branches, aredesigned using geometric principles. Some types ofduct connections used in constructing duct systems areshown in figure 7-43.Sheet metal ducts expand and contract as they heatand cool. Fabric joints are often used to absorb thismovement. Fabric joints should also be used where theduct connects to the air conditioner. Many ducts areinsulated to lower noise and reduce heat transfer. Theinsulation can be on the inside or the outside of theduct. Adhesives or metal clips are commonly used tofasten the insulation to the duct. As we are only brieflydiscussing construction here, you can find construc-tion and fabrication methods in the Steelworker,volume 2. It details design and fabrication of steelductwork.7-43